The present invention relates to protective relay circuits, and more particularly relates to a novel protective relay which may be considered to be a combination distance and overcurrent relay which provides significant advantages on both compensated and uncompensated lines.
Distance relays and static distance relays are well known in the art of protective relaying. For example, reference may be had to U.S. Pat. No. 3,374,399-Dewey and the patents cited therein. Further discussion of protective relaying by means of distance relays may be had by reference to the Art and Science of Protective Relaying, Mason, published by John Wiley & Sons, Inc. (1956). It is well known in the relay art that protective relays have a tendency to pick up for faults farther away than expected under certain conditions. This tendency is well known in the relay art as "overreach". The present invention substantially solves the problem of overreaching where there are series capacitors in the protected line section and also solves the problem of incorrect directional signalling when the series capacitors are located behind the relay location.
In the application of distance relays, the voltage and current in the line serve as inputs. Effectively, the voltage to current ratio is utilized to obtain a signal representative of the impedance from the protective relay sensing means to a fault location. If this signal representative of an impedance value indicates a fault within a predetermined distance of the relay monitoring circuitry, a signal is developed for controlling operation of the circuit breaker trip coil. Problems in the form of the detection of false fault condition signals have arisen when an open circuit has occurred in the circuitry providing the voltage signal. The voltage signal, which is a signal proportional to the line current times the impedance to the fault, acts as a restraining signal restraining the operation of the relay. When this signal disappears, due to an open circuit condition in the monitoring circuitry, such as a blown fuse or the like in the potential supply, the distance relay circuitry may provide a signal indicative of a fault condition. The present invention eliminates this possibility where the load current has not exceeded normal values.
In the past, protective relaying by means of distance relays suffered from a serious dilemma where a series capacitor or capacitors is used in the line for compensation purposes. For example, if a line had an impedance of 10 ohms and a compensating capacitive reactance of -5 ohms, it was necessary to set the reference or replica impedance of the line to a much smaller value than the difference between the line impedance and the capacitor impedance, 5 ohms, or else the distance relay had to be designed to operate very slowly to avoid substantial transient overreach. A further problem occurs where series capacitors are used in a location just behind the relay, that is, in the non-tripping direction. If a fault occurred near the capacitor but on the opposite side to the relay location, there was a good probability that the distance relay would produce an undesired trip output. The present invention also provides a solution to both of these problems. However, the latter problem may be solved by blocking the tripping relay by the prior operation of a blocking relay which looks in the non-tripping direction.
It is well known in the protective relaying art that it is essential to trip the circuit breaker more quickly for faults which occur near the generating source, such faults being referred to as close-in faults. The present invention enables the tripping of the circuit breaker to be effected more quickly on close-in faults thereby enabling the removal of the line containing the faults before it pulls a generator out of step or results in other significant undesirable results.